1. Field of the Invention
The present invention relates to an element for magnetic recording and, more particularly, to a flexible magnetic element practically free from transverse incurvature, and having an excellent aptitude for calendering. The invention also relates to a method for making such an element.
2. The Prior Art
In general, tapes for the magnetic recording of sounds or of images comprise a flexible support, coated on one of its surfaces with one or more layers comprising a dispersion of magnetizable particles, such as gamma-ferric oxide, chromium dioxide, ferrites, or metallic powders, in a suitable binder. As binders, a variety of different organic polymers can be used, e.g., vinyl polymers such as polyvinyl chloride, polyvinyl butyral, or copolymers of vinyl acetate and vinyl chloride; copolymers of vinylidene chloride and acrylonitrile; copolymers of acrylic and/or methacrylic esters; copolymers of styrene and butadiene; terpolymers of acrylonitrile, vinylidene chloride, and maleic anhydride; cellulose derivatives; reticulated or non-reticulated polycondensates, such as polyamides, polyesters, and polyurethanes; or mixtures of these binders with one another.
Often the completed magnetic tapes, in the dry state, have a tendency to assume a transverse incurvature, which becomes greater as the support is thinner, the oxide is finer, and the binder is used in greater amount. This incurvature results mainly from the contraction undergone by the binder or mixture of binders during the drying operation, subsequent to application of the layer of magnetic composition to the support. Consequently, the magnetically coated side of the layer becomes concave and the opposite side becomes convex.
This phenomenon of transverse incurvature is extremely disturbing, both during the subsequent operations of finishing the magnetic tapes and during travel of finished tapes through the recording and reproduction apparatus. In the latter case, the magnetic tapes which display the defect of transverse incurvature, contact only imperfectly the magnetic heads for recording and/or reproduction, which is detrimental to the quality of the recording and/or of the playback or reading, and which may even render the magnetic tape unusable. This phenomenon is particularly detrimental in the case of magnetic tapes designed for the recording of images, due to the relatively high speeds between the heads and the magnetic layer.
Various processes have been suggested to correct this disadvantage of transverse incurvature of magnetic tapes. For example, it has been suggested to avoid transverse incurvature by adding plasticizers to the binder of the magnetic layer, but the incorporation of a large amount of plasticizer changes the properties of the magnetic layer which becomes soft and tends to stick and to soil the magnetic heads. It has also been suggested to incorporate into the magnetic layer inert pigments, such as kaolin or titanium dioxide, but this process has the disadvantage of reducing the cohesion and the adherence of the magnetic layer. It has further been suggested, as described in French Pat. No. 2,035,255, to eliminate the transverse incurvature of magnetic tapes by treating the back of said tapes, completely dried, by chlorinated solvents such as dichlorethane, dichlorethylene, and dichloromethane, but this process has the disadvantage of requiring a supplementary operation during the manufacture of magnetic tapes.
On the other hand, it is known that, particularly in the case of magnetic recording of images, the magnetic tapes must not only be flat, but also display a very smooth surface, so as to assure intimate physical contact of the magnetic tape with the magnetic recording and reproducing heads. In order to obtain such a smooth surface, the magnetic tapes usually are submitted to one or more calendering operations after the magnetic layer has been applied and has dried.